The advantage of the "L" is that it is much easier to use, so if your subject is going to disappear in a few seconds, you can capture it while the USM would still be hunting for focus. If you can be deliberate and take your time, and use a tripod or very fast shutter speed, the USM IQ is actually a tad better.

I agree. I sold the 100mm and got myself a 100mm L, I was not impressed by the IQ (it was good but not much better than the 100mm regular, infact the regular 100mm seemed sharper to me.). I ended up selling the L as well... maybe one day I can afford the 180mm L, I'd really like to try one of those out.

I had the 100L first, traded it cause I didn't use it much. Then bought the 100 non-L on CL. I don't see a difference in IQ, they are both great! I use it for studio product photography so the weather sealing and IS are useless.

I also like using the 100 macro as a walk-around telephoto with basically no minimum focus distance. If I'm too close, well, I just get a detail shot- as opposed to no shot.